In air conditioners and refrigerators, electric compressors with built-in electric motors are used in order to compress refrigerant. These motors for electric compressors employ an embedded magnet electric motor that is provided with a rotor in which a plurality of permanent magnets are embedded at predetermined intervals in the circumferential direction at the outer periphery of a rotor iron core that is formed by laminating magnetic steel plates.
Such an electric motor is provided with a stator and a rotor disposed at the inner side of the stator with a predetermined gap between them, and the rotor has a configuration in which, in a cross-section perpendicular to the axial direction, a plurality of poles, namely, main magnetic pole portions that are provided with magnet-insertion holes for embedding permanent magnets and auxiliary magnetic pole portions (auxiliary salient pole portions) that are constituted of iron core portions between the main magnetic pole portions, are alternately disposed in the circumferential direction. With this configuration, the motor is configured so that rotational torque is gained by utilizing both magnet torque due to magnetic fluxes of the permanent magnets and reluctance torque due to magnetic saliency of the main magnetic pole portions and auxiliary magnetic pole portions.
As shown in FIG. 5, at a main magnetic pole portion 102 of a rotor 101, a non-magnetic portion 104 constituted of a magnetic gap is formed at an end of a permanent magnet 103. In other words, the permanent magnet 103 is inserted into a portion of a magnet-insertion hole 106 formed in the rotor 101, and an opening 105, which is the remaining portion, serves as the non-magnetic portion 104.
This non-magnetic portion 104 is positioned between the main magnetic pole portion 102 and an auxiliary magnetic pole portion 107 that are adjacent to each other in the circumferential direction of the rotor 101, and makes the change of the magnetic-flux density distribution at this position due to the permanent magnet 103 moderate.